1. Field of the Invention
The present invention relates to a magnetic disk substrate for use in a high-density recording/reproducing magnetic disk to be used in a computer or the like.
2. Description of the Related Art
A magnetic disk as a recording medium for a computer is constituted by a substrate and a magnetic film formed on the substrate, and the substrate is required to have the following characteristics.
(1) The substrate must be precisely polished so that a magnetic head can stably move on a disk.
(2) The substrate must be free from cracks or steps which may cause a defect in a magnetic film when the magnetic film is formed on its surface.
(3) The substrate must withstand heating performed during formation of a magnetic film.
(4) The substrate must have a satisfactory hardness which can prevent the substrate from being damaged or abraded when it is brought into contact with a magnetic head.
(5) The substrate must be light in weight and non-magnetic.
As a substrate material which satisfies the above requirements, an aluminum alloy such as an Al-Mg alloy has been conventionally used. In conventional techniques, Ni-P plating or the like is performed to harden the surface of a substrate and to cover an inclusion which causes a magnetic film defect Masahiro Saito et al., Jitsumu Hyomen Gijutsu (Practical Surface Techniques), Vol. 35 (1988), No. 6.
In addition, glass (Hiroyoshi Ishizaki, "Industrial Material", Vol. 35, No. 5), titanium (Published Unexamined Japanese Patent Application Nos. 52-105804, 59-151335, and 1-112521), and the like are developed as the substrate material.
Substrates consisting of the aluminum alloy, glass, and titanium described above, however, respectively have the following problems.
(1) Aluminum Alloy
Since an aluminum alloy is a soft material, the surface of a substrate consisting of the aluminum alloy must be hardened by Ni-P plating as described above. It is, however, difficult to uniformly perform such an electrochemical treatment throughout a wide region on the substrate. In particular, the Ni-P plating easily causes a plating failure. In addition, a magnetic film is generally formed by sputtering, and the substrate is heated during the sputtering. When the Ni-P plating layer is formed on the surface of the substrate, the Ni-P plating layer is crystallized to be magnetic or to easily cause peeling if the substrate temperature exceeds 300.degree. C. upon sputtering. Therefore, the temperature during the sputtering must be limited to be 300.degree. C. or less. Furthermore, a demand has recently arisen for a smaller thickness of a magnetic disk and a higher rotational speed. Since the aluminum alloy essentially has low strength and stiffness, it cannot sufficiently satisfy these requirements.
(2) Glass
Although glass has no problem in heat resistance as a substrate material, it is essentially a brittle material and therefore is easily broken. In addition, when a temperature is increased during sputtering, glass releases gas components, and an impurity in glass is diffused into a magnetic film, thereby degrading the magnetic characteristics of the magnetic film.
(3) Titanium
Although titanium is free from the above drawbacks of the aluminum alloy and glass and therefore expected to be promising as a magnetic disk substrate, the techniques described in the patent applications cited above have the following problems.
That is, Published Unexamined Japanese Patent Application No. 52-105804 discloses a technique of oxidizing or nitriding the surface of Ti to increase its surface hardness, thereby improving polishing properties to obtain good surface conditions and a high abrasion resistance. However, since it is difficult to form such a film having a uniform thickness throughout a wide region, the manufacturing yield is decreased to increase the manufacturing cost. Published Unexamined Japanese Patent Application Nos. 59-151335 and 1-112521 disclose disk substrates consisting of Ti-5Al-2.5Sn, Ti-6Al-4V, and Ti-15V-3Cr-3Al-3Sn alloys. However, since these titanium alloys contain an expensive alloy element at a high concentration, the manufacturing cost is increased. In addition, any of these titanium alloys has poor cold rolling properties (Nishimura; "Kobe Steel Ltd. Technical Reports 32 (1982)", No. 129, page 44) and therefore causes edge cracking upon cold rolling. Since a thin plate such as a disk substrate is broken by cracks, it is practically impossible to manufacture a disk substrate consisting of the titanium alloy by cold rolling. For this reason, a thin plate such as a disk substrate consisting of an alloy of this type is manufactured by hot rolling in accordance with a pack rolling method (Suenaga; "NKK Technical Reports (1987)", No. 127, page 37). A disk substrate manufactured by this method, however, is very expensive.
As described above, no magnetic disk substrate which can satisfy the market's needs in terms of both performance and manufacturing cost has been developed yet.